1. Field of the Invention
The present invention relates to a semiconductor storage device.
2. Description of the Related Art
In recent years, a metal oxide having semiconductor characteristics (hereinafter referred to as an oxide semiconductor material) has attracted attention as a material for an active layer of a transistor. A transistor including an oxide semiconductor material as a material for an active layer can be manufactured through a process similar to that of a transistor including amorphous silicon as a material for an active layer and has higher mobility than the transistor including amorphous silicon. Therefore, the transistor including an oxide semiconductor material as a material for an active layer has been expected to replace or excel the transistor including amorphous silicon as a material for an active layer. For example, the transistor including an oxide semiconductor material as a material for an active layer has been expected to be used as a transistor provided in each pixel in an active matrix display device.
Further, the transistor including an oxide semiconductor material for an active layer has small off-state current. Thus, a potential of a node (the amount of charge held in the node) which is brought into a floating state when the transistor is turned off can be held for a long time. Accordingly, it is expected that a semiconductor storage device is formed using the transistor. For example, Patent Document 1 discloses a dynamic random access memory (DRAM) including the transistor in a memory cell.
The transistor including an oxide semiconductor material for an active layer is formed by deposition and processing of a variety of thin films. Therefore, the transistor can be formed in a region overlapping with a region where another semiconductor element exists. For example, Patent Document 2 discloses a semiconductor device which includes two kinds of transistors stacked: a transistor (transistor in a lower portion) in which an active layer is formed using a semiconductor substrate and a transistor (transistor in an upper layer) in which an active layer is formed using an oxide semiconductor material.